In the past, in the preparation of films useful for packaging purposes, heat sealable coatings, such as acrylic coatings, were coated on one side of the film substrate and another heat sealable coating, such as polyvinylidene chloride (PVDC) was coated on the other side. The acrylic coated side was generally the outside of the film, the side in direct contact with the hot sealer surfaces, where good hot slip and jaw release characteristics are required. The PVDC coating was usually on the inside of the film and provided the high seal strength, good hot tack characteristics and barrier properties required for such packaging. These heat sealable coatings have glass transition temperatures ("Tg"s) which are higher than room temperature. Such a coated film is disclosed in U.S. Pat. No. 4,403,464.
Similarly, U.S. Pat. No. 4,456,741 discloses heat sealable terpolymer compositions useful as pressure-sensitive adhesives for use with, for example, backing materials including paper, polyester film and foamed polymers. The terpolymer heat sealable pressure-sensitive adhesive composition comprises butyl acrylate, N-vinyl-2-pyrrolidinone and styrene. Other heat sealable coatings are disclosed in U.S. Pat. No. 3,696,082; and East German Patent No. DD-146,604.
In packaging products which are sensitive to heat, such as candies, chocolates, ice cream and the like, in plastic film or paper packages, the use of heated elements must be avoided in order to prevent melting of the products. Therefore, the use of heat sealable coatings to package heat sensitive products has presented serious difficulties often requiring isolation of the product from the heated elements. Cold sealable pressure-sensitive adhesives were developed which did not require the use of a heated element to seal the packages. However, these adhesives had high surface tack characteristics which made them adhere to uncoated surfaces of the packaging film, making these adhesives difficult to use due to the resulting blocking (i.e. sticking) of the film.
Findley 207-939, a polyisoprene adhesive manufactured by Findley Adhesive, Inc., is a cold sealable pressure-sensitive adhesive coating which exhibits good crimp seal strength on oriented polypropylene film and has a Tg of -1.6.degree. C. This adhesive has a high surface tack which often results in blocking of the packaging film.
Another such pressure-sensitive adhesive composition is disclosed in U.S. Pat. No. 4,500,683 (Hori et al.). The pressure sensitive adhesives of this patent contain as a polymer component an addition polymerization polymer of an acrylate or methacrylate polymer and one or more ethylenically unsaturated monomers, such as acrylonitrile, capable of forming homo- or copolymers having a glass transition temperature of at least 273.degree. k. The composition is made by solution polymerization or bulk polymerization. This composition forms a viscous adhesive composition which is tacky at room temperature, thus presenting the blocking problems when used on packaging films.
Various other pressure sensitive films have been disclosed. U.S. Pat. No. 2,795,564 (Conn et al.) discloses quick tack adhesive films made by emulsion polymerization of a soft-polymer- forming monomer of alkylacrylate, an .alpha., B-unsaturated monovinylidene carboxylic acid and a hard-polymer-forming monomer such as acrylonitrile.
Similar emulsion polymers have been described for different uses in U.S. Pat. No. Re 24,906; (Ulrich) and in U.K. Patent No. 1,003,318 (Smith et al.).
U.S. Pat. No. Re 24,906 discloses a pressure sensitive adhesive which adheres to paper and is cohesive. This composition may have high blocking properties since a low adhesion liner or coating is suggested for purposes of protecting the adhesive surface from forming a permanent bond.
U.K. Patent No. 1,003,318 discloses an alkali-soluble emulsion polymerized copolymer as an adhesive surface coating.
Acrylic-based formulations are used as protective outside coatings for packaging films used in wrapping products. These acrylic-based coatings improve machineability, printability and flavor and aroma protection. However, the known low blocking pressure sealable coatings such as those described in U.S. Pat. No. 5,070,164 (Min et al.) tend to block (stick) to acrylic-based formulations. This blocking causes serious difficulties during packaging and material handling.
Thus, the related art has disclosed heat sealable coatings and pressure-sensitive, cold sealable adhesives useful in the packaging art. The related art has not, however, disclosed a cold sealable, pressure-sensitive cohesive formulation, which is cohesive only when placed under pressure in contact with other cold sealable cohesive coated surfaces and does not block to acrylic-based outside formulations.